This invention relates to a mobile wireless communication system including a wireless base station and wireless communication terminals, and more particularly to a technique for packet scheduling.
In the downlink communication (from a base station to a mobile station) in the cdma2000 1x-EV DO system, the base station divides time into units (slots) of 1/600 second, and the base station communicates only with a single mobile station in one slot (channel) at a certain frequency and switches the communicating station for each channel to communicate with a plurality of mobile stations.
Mobile stations receive pilot signals from the base station, estimate channel states from the pilot signals, and send the results of channel state estimation (channel state information) to the base station. The base station determines, on the basis of the received channel state information, to which mobile station the next channel should be allocated for packet transmission. The allocation of channels is called packet scheduling. The packet scheduling will be described referring to FIG. 30.
FIG. 30 shows a conventional channel schedule table, where the vertical axis shows time and the horizontal axis shows frequency. The frequency band is divided into carriers F1 to F4 and a certain length of time is divided into slots S1 to S8, so as to form time- and frequency-divided channels.
A mobile station A determines which of the carriers F1 to F4 it should use for communication. Specifically, the mobile station A monitors a reference first carrier stored in SRAM of the mobile station A and judges from broadcast information whether the first carrier is filled up by other mobile stations. When the first carrier is not filled, the mobile station A sends its own control information to the base station to register its position.
On the other hand, when the first carrier is filled up, the mobile station A monitors another, second carrier. Similarly, when the second carrier is filled, the mobile station A monitors a third carrier, so as to determine which carrier to use for communication.
In this example, the mobile station A determines to communicate on the carrier F1. Then, the mobile station A estimates channel states of the individual slots S1 to S8 of the carrier F1 and sends the channel state information to the base station. As stated earlier, the base station allocates channels on the basis of the channel state information from each mobile station. As for the mobile station A, the base station judges that the channel state of the slot S3 is good and allocates (schedules) the channel of the slot S3 of the carrier F1 for the mobile station A.
The mobile station A may communicate using a plurality of carriers. For example, JP 2003-9240 A describes a technique in which a mobile station A communicates using a plurality of carriers. The technique will be described referring to FIG. 31.
FIG. 31 shows a conventional channel schedule table, where the vertical axis shows time and the horizontal axis shows frequency.
The base station communicates with a plurality of mobile stations using carriers F1 to F8. Here, the base station and the mobile station A communicate at a low rate.
First, the base station determines to allocate three carriers to the mobile station A for communication. The base station may allocate the plurality of carriers F1, F4, and F8 to the mobile station A so that the frequency differences between the allocated carriers exceed a predetermined value. After that, the base station sends packets to the mobile station A using the carrier F1, the carrier F4, and the carrier F8. In this case, the mobile station A sends, to the base station, channel state information for each of the slots S1 to S8 on the carriers F1, F4, and F8. The base station performs scheduling on the basis of the channel state information from each mobile station. The base station judges that the channel states of the channel of the slot S3 of the carrier F1, the channel of the slot S4 of the carrier F4, and the channel of the slot S7 of the carrier F8 are good and schedules these channels for the mobile station A. The base station then sends packets to the mobile station A over the scheduled channels.